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between these residues. This lowers the entropy or raises the energy of the system.
Once the tension is released on the fiber, the hydrophobic interactions resume.
These are further supported by the surrounding
-spirals. This results in the return
of the fiber to its higher entropy/lower energy state and original shape (Rosenbloom
et al. 1993 ; Urry and Parker 2002 ).
b
9.3 Structural Consequences of Elastin Degradation
Elastin is fairly resilient; however, some proteases can degrade it. The degradation
of the elastic fiber is pathologic and results in disease in the various organs. We will
briefly focus on elastic fiber degradation in the lung (emphysema), large vessels
(aortic aneurysm), and skin (solar elastolysis).
In the lungs, elastin can be found in the arterial vessels of the lung arranged as
sheets, surrounding the respiratory bronchioles and alveolar ducts in a helical
fashion, and lining the walls of the alveoli (Dunsmore 2008 ). Elastin provides the
lungs with their ability to stretch and relax. The hallmark disease involving elastin
cleavage in the lung is emphysema.
When an individual smokes, inflammatory cells are recruited including macro-
phages, neutrophils, and lymphocytes. As these cells release proteinases, they
destroy the basement membrane and matrix resulting in the coalescence of alveoli
into larger ones giving the emphysema phenotype (Shapiro and Ingenito 2005 ). The
destruction of alveoli and elastic fibers decreases the elastic recoil making the lungs
much more compliant. Since exhalation depends on the elastic recoil of the lung,
the loss of elastic recoil causes a decreased ability to expire gas during exhalation.
This decreased ability to expire gas is the “obstructed” phenotype characteristic of
emphysema. Furthermore, the smaller airways are in contact with the alveoli and
are tethered open during respiration. As the elastin and matrix are cleaved, there is
small airway closure and collapse, which may further exacerbate the obstructive
phenotype. These situations may result in air retention at the end of exhalation,
which puts the chest wall and diaphragms at mechanical disadvantage and increases
the work of breathing. Furthermore, increased work of breathing, loss of alveoli,
and retained air in the lungs can significantly alter ventilation throughout the lungs
resulting in inconsistent regions of gas exchange. Restoration of the elastic recoil of
the lung could potentially reverse this process, which is the goal of lung volume
reduction surgery and bronchoscopic lung volume reduction.
Elastic fibers are required for large arteries to withstand the pulsatile pressure of
systole, absorb the energy, and return it so that there is perfusion pressure through-
out diastole. The large arteries are composed of three main layers: the intima
(innermost layer composed of endothelial cells), media (middle layer composed
mostly of smooth muscle cells), and adventitia (outer layer which is mostly
acellular). The intima and media are separated by the internal elastic lamina and
the media and adventitia are separated by an external elastic lamina (Schoen and
Cotran 1999 ). Furthermore, in the media there are layers of elastin associated with
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